GB2329732A

GB2329732A - Automatic voltage level control circuit having select time constants

Info

Publication number: GB2329732A
Application number: GB9819947A
Authority: GB
Inventors: Fuad Haji Mokhtar; Nazira Bt Khamar; Macwien Krishnamurth Annamalai
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1997-09-26
Filing date: 1998-09-15
Publication date: 1999-03-31
Anticipated expiration: 2018-09-15
Also published as: MY118209A; GB2329732B; DE19843767A1; GB9819947D0; BR9803589A; KR19990030150A; KR100317212B1

Abstract

An automatic voltage level control circuit (1) comprising an automatic voltage level control input (2) and a comparator (3) having an output (4), a reference input (5) and a feedback input (6). There is also an amplifier (7) having a control input coupled to the output (4), a feedback output (9) for providing a signal associated with a power level being supplied by the amplifier (7), the feedback output (9) being coupled to the feedback input (6). There are at least two selectable time constant networks coupled to both the reference input (5) and the automatic voltage level control input (2) to thereby allow for at least two time constants to be electrically associated with the reference input (5). There is also an automatic switch (10) electrically associated with the time constant networks to thereby allow for automatic switching from one time constant to another time constant when the automatic switch (10) detects a voltage level that is above a threshold level.

Description

1 2329732 AUTOMATIC VOLTAGE LEVEL CONTROL CIRCUIT

FIELD OF THE INVENTION

This invention relates to an automatic level control circuit that is particularly useful for, but not necessarily limited to, controlling a Radio Frequency Power Amplifier associated with a voltage controlled oscillator.

BACKGROUND ART

In general, Radio Frequency Power Amplifiers are designed to operate at different power levels and in use they are controllable by control voltages having different rates of change. For instance, when selecting either low power and high power applications for such amplifiers, the rate of change of reference voltage is faster in high power applications as compared to the low power application. Accordingly, the control voltage steady state condition can occur at di fferent times for high power applications as compared to low power applications. As a result, when high power is required the control voltage can prematurely reach steady state causing abrupt frequency pulling of a Voltage Controlled Oscillator driving the Radio Frequency Power Amplifier. This in turn can create transitional frequency shifts that are contrary to industry standards such as the ETS300 test specification. However, by slowing down both rates of change to avoid abrupt transitions, timing requirements of the MPT1327 system standard could be breached when low power mode is selected.

Further to the above, different Radio Frequency Power Amplifiers when operating at high power have a calibrated control voltage to provide the desired power output. In use, the steady state condition for the control voltage will occur at different times depending upon the initiation of a ramped reference voltage. Accordingly, this can cause similar problems to those described above.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome or alleviate at least one of the problems associated with controlling Radio Frequency Power Amplifiers.

2 According to one aspect of the invention there is provided an automatic voltage level control circuit comprising:

an automatic voltage level control input; a comparator having a comparator output, a reference input and a feedback input; an amplifier having a control input coupled to said comparator output, a feedback output for providing. a signal associated with a power level being supplied by said amplifier, said feedback output being coupled to said feedback input; at least two selectable time constant networks coupled to both said reference input and said automatic voltage level control input to thereby allow for at least two time constants to be electrically associated with said reference input; automatic switching means electrically associated with said time constant networks to thereby allow for automatic switching from one time constant to another time constant when said automatic switching means detects a voltage level that is above a threshold level.

Preferably, said automatic switching means may be one or more diodes. The diodes may suitably be Schottky diodes, conventional diodes, zener diodes or any other suitable type of diodes.

Suitably, said time constant networks may include Resistor Capacitor networks, Inductor Capacitor networks or combinations thereof.

Preferably, one of said time constant networks may be a first Capacitor and Resistor in series therewith, and another of said time constant networks is a second Capacitor insertable in parallel with said first Capacitor by said switching means.

Preferably, said amplifier can be a radio frequency amplifier. There may suitably be a voltage controlled oscillator coupled to said radio frequency amplifier for the driving thereof.

Suitably, there may be a rate of change detector electrically associated with said comparator output and said reference input, wherein said rate of change detector is adapted to temporarily alter a rate of change of a signal at said reference input when a signal at said comparator output is above both a threshold level and a threshold rate of change.

3 Preferably, said rate of change detector may include a pull down switch for temporarily coupling said reference input to ground.

Suitably, said rate of change detector may also include resistor capacitor circuitry coupled to said pull down switch.

Preferably said pull down switch may be a transistor having a base or gate input coupled to said resistor capacitor circuitry.

The said automatic voltage level control input can be suitably coupled to a signal supply source for providing at least two ramped voltages of different amplitudes and rate of changes.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be readily understood and put into practical affect, reference will now be made to a preferred embodiment illustrated in the accompanying drawings in which:

Fig. 1. is an embodiment of an automatic voltage level control circuit in accordance with the present invention; Fig.2 illustrates low and high ramped reference voltages applied to the automatic voltage level control circuit of Fig. 1; FIG. 3 illustrates a problem associated with control voltages of prior art automatic voltage level control circuits when using the ramped reference voltages of Fig. 2; Fig. 4 illustrates modified ramped reference voltages provided by the automatic voltage level control circuit of Fig. 1; Fig. 5 illustrates modified control voltages provided by the automatic voltage level control circuit of Fig. 1; Fig. 6 illustrates examples of ramped reference voltages of different magnitudes that should be applied automatic voltage level control circuit of Fig. 1, these magnitudes being dependent upon the control voltage required to provide a required power output;.

Fig. 7 illustrates a problem associated with control voltages due to the possible different ramped reference voltages of Fig. 6 that may be applied to prior art automatic voltage level control circuits;

Fig. 8 Fig. 4 illustrates modified ramped reference voltages provided by the automatic voltage level control circuit of Fig. 1; and Fig. 9 illustrates modified control voltages provided by the automatic voltage level control circuit of Fig. 1.

4 DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring to Fig. 1 there is illustrated an automatic voltage level control circuit 1 comprising an automatic voltage level control input 2 and a comparator 3. The comparator has a comparator output 4 a reference input 5 (Vrel) and a feedback input 6. The automatic voltage level control circuit 1 also has a radio frequency power amplifier 7, two selectable time constant networks T1, T2 and an automatic switching means in the form of a diode network 10. The time constant networks T1J2 are coupled to both the reference input 5 and automatic voltage level control input 2 The amplifier 7 has a control input 8 (Vctri) coupled to the comparator output 4. The amplifier 7 also has a feedback output 9 (Vfb) for providing a signal associated with a power level supplied at an output OUT of amplifier 7, this feedback output 9 is coupled to feedback input 6 of comparator 3. The time constant network Tl comprises a first capacitor Cl in series with resistor Rl therefore the associated time constant Tcl is R1.Cl. The second time constant network T2 comprises resistor Rl in series with the parallel combination of capacitors Cl, C2 therefore the associated time constant Tc2 is R1.(Cl+C2).

At the comparator output 4, there is a rate of change detector 19 comprising a pull down transistor Q1 having a base B coupled to a resistor capacitor circuitry comprising resistors R2, R3 and capacitor C3. A collector C of transistor Q1 is coupled to Reference Input 5 (Vref) and an emitter E of transistor Q1 is coupled to ground. One side of the resistor R3 is also coupled to ground, the other side of resistor R3 is coupled to both base B and one side of a parallel network of resistor R2 and capacitor C3. The other side of the parallel network of resistor R2 and capacitor C3 is coupled to comparator output 4 (Vctri). Coupled to the amplifier 7 is a voltage controlled oscillator 20 to provide driving thereof. Further, the automatic voltage level control input 2 can be suitably coupled to a digital to analogue converter 21 for providing two or more ramped voltages of different amplitudes and rates of changes.

In use, as illustrated in Fig. 2, the digital analogue converter 21 can provide ramped voltages to reference input 5 (Vref), these ramped voltages correspond to either a low or high power control signal (Vctri) supplied to amplifier 7 and they have different amplitudes and inherent different rates of change. As illustrated in Fig. 3, prior art automatic level control circuits have a disadvantage of the high power control signal supplied to control input 8, when compared with the low power control signal, prematurely reaching a steady state condition. This therefore may cause abrupt frequency pulling of the Voltage Controlled Oscillator 20 which in turn may cause undesirable transitional frequency shifts which can be contrary to industry standards. Accordingly, in use, the present invention will modify the rates of change of the reference input voltages as, illustrated in Fig. 4. This is achieved by automatically switching from time constant network Tl to time constant network T2 when the diode network 10 detects a voltage level that is above a voltage threshold level. In the embodiment desc(ibed in Fig. 1, this threshold level is approximately 1.44 Volts (the forward breakdown voltage of four conventional series coupled Schottky diodes). If the low power ramped voltage is supplied to input 2, the voltage across diode network 10 will be below the voltage threshold level and therefore time constant network time constant Tl having time Tcl is coupled to input 2. However, if the high power ramped voltage is supplied to input 2, the voltage across diode.network 10 will be go above the voltage threshold level therefore forward biasing diode network 10 resulting in capacitor C2 being inserted in parallel with capacitor Cl. Accordingly, time constant network T2 having time Tc2 will be coupled to input 2. As a result, as illustrated in Fig. 5, a control voltage Vctd for both high power and low power ramped voltages can reach steady state the same time by suitable selection of capacitors Cl, C2. It should be noted that the use of the diode network 10 offers an advantage of smooth' switching due to the varying internal resistance of the diodes and therefore a substantially smooth input signal to input 2 is achieved when a high power ramped voltage is provided from the digital to analogue converter 21. It should also be note that diode network 10 also allows a similar advantage dudng discharge of capacitor C2.

In certain circumstances, to get the same power output from different amplifiers 7, the reference voltage (Vref) applied to input 2 will need to be of a different magnitude as illustrated in Fig. 6. Unfortunately, as shown in Fig. 7.9 prior art automatic level control circuits do not provide compensation for such requirements. Consequently, the steady state condition for the control voltage (Vctri) supplied to control input 8 will occur

6 at different times from initiation depending upon the reference voltages (Vref) supplied to the input 5. Accordingly, the present invention by use of the rate of change detector 19 obviates this disadvantage by detecting the rate of change of control voltage (Vctri) which when above a threshold level LT will turn on transistor Q1 for a short duration until the rate of change falls below this threshold level LT. This therefore results in input 2 being temporarily shorted to ground which temporarily alters the rate of change of the control voltage (Vctri). In this regard, the threshold level LT in this embodiment is a current 1 applied to the base B of transistor Q1, wherein I=C3.(dVctildt). As will be apparent to a person skilled in the art, in use the rate of change detector 19 results in the reference voltage (Vref) at input 2 being modified as illustrated in Fig. 8 due to input 2 being temporarily coupled to ground for a short period 8 and in response thereto a modified control voltage (Vctri) occurs at control input 8 as illustrated in Fig. 9.

Advantageously, as can be seen in Figs. 2 to 9, the present invention overcomes or at least alleviates at least one of the problems associated with controlling Radio Frequency Power Amplifiers.

Although the invention has been described with reference to a preferred embodiment it is to be understood that the invention is not restricted to the embodiment described herein.

7

Claims

WE CLAIM:

1 An automatic voltage level control circuit comprising: an automatic voltage level control input; a comparator having a comparator output, a reference input and a feedback input; an amplifier having a control input coupled to said comparator output, a feedback output for providing a signal associated with a power level being supplied by said amplifier, said feedback output being coupled to said feedback input; at least two selectable time constant networks coupled to both said reference input and said automatic voltage level control input to thereby allow for at least two time constants to be electrically associated with said reference input; automatic switching means electrically associated with said time constant networks to thereby allow for automatic switching from one time constant to another time constant when said automatic switching means detects a voltage level that is above a threshold level.

2. An automatic voltage level control circuit as claimed in claim wherein said automatic switching means is one or more diodes.

3. An automatic voltage level control circuit as claimed in claim 2, wherein said time constant networks includes Resistor Capacitor networks, Inductor Capacitor networks or combinations thereof.

4. An automatic voltage level control circuit as claimed in claim 3, wherein, one of said time constant networks is a first Capacitor and Resistor in series therewith, and another of said time constant networks is a second Capacitor insertable in parallel with said first Capacitor by said switching means..

5. An automatic voltage level control circuit as claimed in claim 1, wherein said amplifier is a radio frequency amplifier.

8

6. An automatic voltage level control circuit as claimed in claim 5, wherein there is a voltage controlled oscillator coupled to said radio frequency amplifier for the driving thereof.

7. An automatic voltage level control circuit as claimed in claim 1, wherein there is a rate of change detector electrically associated with said comparator output and said reference input, wherein said rate of change detector is adapted to temporarily alter a rate of change of a signal at said reference input when a signal at said comparator output is above both a threshold level and a threshold rate of change.

8. An automatic voltage level control circuit as claimed in claim 7, wherein said rate of change detector includes a pull down switch for temporarily coupling said reference input to ground.

9. An automatic voltage level control circuit as claimed in claim 8, wherein said rate of change detector includes resistor capacitor circuitry coupled to said pull down switch.

10. An automatic voltage level control circuit as claimed in claim 8, wherein said pull down switch is a transistor having a base or gate input coupled to said resistor capacitor circuitry.

11. An automatic voltage level control circuit as claimed in claim 1, wherein said automatic voltage level control input is coupled to a signal supply source for providing at least two ramped voltages of different amplitudes and rate of changes.